glycogen, also referred to as animal starch,
and various mixtures of amylose and
amylopectin. Amylose is a linear polymer
of glucose linked by 1–4 glycosidic bonds.
There are typically 1000–2000 glucose
monomers per molecule of amylose.
Amylopectin also is a polymer made solely
of glucose. It differs from amylose because
it is branched and it is larger. The general
structure of amylopectin resembles that of
a tree. There is a single trunk with a reduc-
ing end (free carbon 1 hydroxyl) elongated
with other glucose monomers attached by
1–4 glycosidic bonds. From this single
trunk, branch points are formed by 1–6
glycosidic bonds. The branches are
elongated by glucose monomers linked by
1–4 glycosidic bonds. On average, there
are branch points every 20–25 glucose
residues. There may be up to 200,000
glucose monomers per molecule of amylo-
pectin. Glycogen is similar to amylopectin
but is more branched. Glycogen contains a
branch point approximately every 10–12
glucose residues. There is no reducing end
on glycogen. The ‘trunk’ of glycogen is
anchored on a specific glycoprotein,
glycogenin.
Starches make up 700 g kg^1 of the
DM of cereal grains, and 300 g kg^1 of the
DM of fruits, roots and tubers, and are the
principal storage polysaccharide of tropical
grasses. Starches, therefore, are a principal
constituent of the diet of many farm
animals, and at least potentially are
digestible by enzymes produced by
animals. The properties of the starch are
determined primarily by the proportions of
amylose and amylopectin in the starch, the
degree of crystallinity of the starch and the
degree of interaction of starch and protein.
This is obvious to the baker who will select
different types of flour (starch) depending
on whether he is making bread, pastries or
pizza crust. Digestion is also influenced by
the type of starch. Starches may be rapidly
and completely digested, slowly but
completely digested, partially digested or
resistant to digestion. Knowledge of these
properties and how the properties can be
altered by processing is important in
formulating diets.
Fructans are a second group of intra-
cellular storage polysaccharides. Fructans
are polymers of fructose attached by 2–6
(levans) or 2–1 (inulins) to each other and
to a sucrose primer. Fructans, therefore,
have no reducing end. There are 30
fructose monomers in a molecule of
fructan. The fructans are the principal
storage polysaccharide of temperate
grasses. Levans are typical of grasses and
are more soluble than inulins. Fructans are
soluble in water and are digested rapidly
by enteric microorganisms but are not
digested by digestive enzymes produced by
animals. The glycosidic linkages of
fructans are hydrolysed easily by acid.
This, along with their water solubility, may
include fructans in the intracellular soluble
carbohydrate category.
Cell wall carbohydrates are also
referred to as structural carbohydrates.
There are four major types of carbohydrates
in this group: cellulose; hemicellulose;
pectins; and gums. All of the carbohydrates
in this group require microbial enzymes for
digestion. Cellulose is a linear polymer of
glucose linked by 1–4 glycosidic bonds.
Cellulose is the most abundant organic
compound on earth. There are 10,000
glucose monomers per molecule of
cellulose. The 1–4 linkages of cellulose
versus the 1–4 linkages of amylose cause
marked differences in the properties of the
two polymers. Cellulose has a flat, straight
structure. This allows cellulose molecules
to align tightly together in straight, parallel
rows to provide structural rigidity. Despite
the rather simple chemical definition of
cellulose, the nutritional characteristics of
cellulose vary from indigestible to com-
pletely digestible by enteric microbes
independently of a host. Cellulose interacts
with other cell wall components such as
hemicellulose, lignin, pectin, cutin and
minerals to various extents. The extent and
nature of the interactions alter the nutri-
tional characteristics of the cellulose.
Hemicellulose is more heterogeneous
than cellulose, with the overall composi-
tion of hemicellulose differing from one
plant species to another. Generally, the
base polymer is xylose linked by 1–4Glucose Availability and Associated Metabolism 123